Abstract: The molecular mechanisms underlying adaptation to physical exertion and racing stress in horses remain incompletely understood. Peripheral blood transcriptomics offers a minimally invasive method to monitor systemic responses to exercise and identify biomarkers of adaptation or overload. Objective: To evaluate transcriptomic changes in peripheral blood of racehorses during different phases of training and competition and to identify molecular markers of physiological adaptation and race-induced stress. Methods: Prospective transcriptomic profiling of trained racehorses across three exercise conditions. Methods: Forty racehorses (29 Arabian, 11 Thoroughbred) were sampled before (p0) and after (p1) exercise at three stages: initial training (T1), mid-season training (T2) and racing (R). RNA-seq was performed, followed by differential expression analysis (DESeq2) and pathway enrichment (g:Profiler, DAVID). Identified differentially expressed genes were integrated into STRING protein-protein interaction condition-exclusive subnetworks (Merge tool in Cytoscape) to compare the transcriptomes between conditions. Results: Distinct molecular programmes were identified at each stage. At T1, immediate-early response genes such as FOS, FOSB and HSPA6 were strongly up-regulated, reflecting acute stress and immune activation. At T2, immune-related transcripts (KLRD1, CCL4, PRF1) remained enriched, but genes linked to remodelling and adaptation, including DNAJA1, HSPH1 and CXCR4, became prominent, suggesting a shift toward recovery and regulatory processes. Post-race, chemokines such as CCL5 and stress markers (HSP90 family, JUN) were highly induced, accompanied by widespread transcriptomic divergence and down-regulation of certain immune regulators (IL18, ARHGAP44), indicating both heightened innate activation and transient immune suppression. Conclusions: Transcriptomic profiling was limited to peripheral blood, which may not reflect tissue-specific responses. Only three sampling points were included, potentially overlooking transient transcriptomic changes. Conclusions: Transcriptomic dynamics in blood reflect the transition from early immune activation (T1), through adaptation (T2), to stress-related activation post-race (R). This approach offers promising molecular biomarkers for monitoring training adaptation and detecting physiological overload in equine athletes. Unassigned: Die molekularen Mechanismen, die der Anpassung an körperliche Anstrengung und Rennstress bei Pferden zugrunde liegen, sind noch nicht vollständig geklärt. Die Transkriptomik des peripheren Blutes bietet eine minimalinvasive Methode zur Überwachung systemischer Reaktionen auf körperliche Belastung und zur Identifizierung von Biomarkern für Anpassung oder Überlastung. Unassigned: Bewertung der transkriptomischen Veränderungen im peripheren Blut von Rennpferden während verschiedener Trainings‐ und Wettkampfphasen und Identifizierung molekularer Marker für physiologische Anpassung und rennbedingten Stress. Methods: Prospektive Transkriptom‐Profilierung von trainierten Rennpferden unter drei Trainingsbedingungen. Methods: Vierzig Rennpferde (29 Araber, 11 Vollblüter) wurden vor (p0) und nach (p1) dem Training in drei Phasen untersucht: zu Beginn des Trainings (T1), in der Mitte der Saison (T2) und während des Rennens (R). Es wurde eine RNA‐Sequenzierung durchgeführt, gefolgt von einer Differentialexpressionsanalyse (DESeq2) und einer Signalweg‐Anreicherung (g:Profiler, DAVID). Identifizierte DEGs wurden in STRING PPIs bedingungsspezifische Subnetzwerke integriert (Merge‐Tool in Cytoscape), um die Transkriptome zwischen den Bedingungen zu vergleichen. Unassigned: In jeder Phase wurden unterschiedliche molekulare Programme identifiziert. Bei T1 waren sofortige Frühreaktionsgene wie FOS, FOSB und HSPA6 stark hochreguliert, was auf akuten Stress und eine Immunaktivierung hindeutet. Bei T2 blieben immunbezogene Transkripte (KLRD1, CCL4, PRF1) angereichert, aber Gene, die mit Umbau und Anpassung in Verbindung stehen, darunter DNAJA1, HSPH1 und CXCR4, traten in den Vordergrund, was auf eine Verlagerung hin zu Erholungs‐ und Regulationsprozessen hindeutet. Nach dem Rennen waren Chemokine wie CCL5 und Stressmarker (HSP90‐Familie, JUN) stark induziert, begleitet von einer weitreichenden transkriptomischen Divergenz und einer Herunterregulierung bestimmter Immunregulatoren (IL18, ARHGAP44), was sowohl auf eine verstärkte angeborene Aktivierung als auch auf eine vorübergehende Immunsuppression hindeutet. Unassigned: Die Transkriptom‐Profilierung beschränkte sich auf peripheres Blut, was möglicherweise nicht die gewebespezifischen Reaktionen widerspiegelt. Es wurden nur drei Probenentnahmepunkte berücksichtigt, wodurch möglicherweise vorübergehende transkriptomische Veränderungen übersehen wurden. Unassigned: Die Transkriptomdynamik im Blut spiegelt den Übergang von der frühen Immunaktivierung (T1) über die Anpassung (T2) bis hin zur stressbedingten Aktivierung nach dem Rennen (R) wider. Dieser Ansatz bietet vielversprechende molekulare Biomarker für die Überwachung der Trainingsanpassung und die Erkennung physiologischer Überlastung bei Sportpferden.
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Overview
This study examined how gene expression in the blood of racehorses changes during different training phases and competitive racing, aiming to identify molecular markers that reflect physiological adaptation and stress.
Researchers used RNA sequencing to capture transcriptomic profiles before and after exercise at three key stages, revealing distinct immune and stress response patterns linked to training adaptation and race-induced strain.
Background and Objective
The biological mechanisms by which horses adapt to physical exertion and the stress of racing are not fully understood.
Peripheral blood transcriptomics — analyzing RNA from blood samples — serves as a minimally invasive method to monitor systemic responses to exercise.
This approach helps identify biomarkers indicating whether a horse is adapting well or experiencing overload from training and racing stress.
The main goal was to evaluate changes in gene expression in racehorses’ blood during different training phases and competitive racing to pinpoint molecular markers of physiological adaptation and stress.
Methods
Forty trained racehorses were studied, including 29 Arabian and 11 Thoroughbred horses.
Blood samples were taken before (p0) and after (p1) exercise at three stages:
Initial training phase (T1)
Mid-season training phase (T2)
During actual racing events (R)
RNA sequencing (RNA-seq) was performed on these samples to profile gene expression.
Differential expression analysis was conducted using the DESeq2 tool to identify genes whose expression levels changed significantly between conditions.
Pathway enrichment analysis using tools like g:Profiler and DAVID identified biological processes and pathways associated with these genes.
Identified differentially expressed genes were integrated into condition-specific protein-protein interaction networks using STRING and visualized with Cytoscape’s Merge tool to compare transcriptomic profiles across the three conditions.
Key Findings
Initial Training (T1):
Strong up-regulation of immediate-early response genes (e.g., FOS, FOSB, HSPA6) indicating acute stress and immune system activation immediately following exercise.
Mid-Season Training (T2):
Immune-related transcripts such as KLRD1, CCL4, and PRF1 remained elevated, showing ongoing immune involvement.
Genes linked to tissue remodeling and adaptation—including DNAJA1, HSPH1, and CXCR4—became more prominent, suggesting a shift toward recovery, repair, and regulatory processes.
Post-Race (R):
Increased expression of chemokines like CCL5 and stress markers belonging to the HSP90 family and JUN indicated heightened stress response.
There was widespread divergence in gene expression profiles compared to training phases.
Some immune regulators (e.g., IL18, ARHGAP44) were down-regulated, suggesting transient immune suppression following intense exertion.
This combination points to both increased innate immune activation and a temporary dampening of parts of the immune system post-race.
Interpretation and Conclusions
Peripheral blood transcriptomics captures systemic adaptations of the immune system over a horse’s training cycle:
An initial acute immune activation immediately after beginning training.
A transitional phase focusing on recovery, tissue remodeling, and regulated immune activity as training progresses.
Strong stress responses and shifts in immune balance following race exertion.
The study highlights promising molecular biomarkers that could be used to monitor whether horses are adapting well or are at risk of physiological overload during training and racing.
Limitations include restricting analysis to peripheral blood, which might not fully represent gene expression dynamics in muscle or other tissues directly involved in exertion.
Only three sampling points were assessed, potentially missing more transient or immediate gene expression changes that occur during or shortly after exercise.
Future studies could incorporate more time points and tissue types to build a comprehensive picture of the molecular processes underlying equine athletic performance and recovery.
Significance
This research provides a molecular framework to understand how systemic immune and stress responses evolve throughout a racehorse’s training and competitive season.
It supports the use of blood-based transcriptomic biomarkers for non-invasive monitoring of horse health, training adaptation, and early detection of overtraining or stress-induced immune suppression.
Such insights can ultimately contribute to better management and welfare of equine athletes by guiding training regimens and recovery strategies based on molecular evidence.
Cite This Article
APA
Dąbrowska I, Grzędzicka-Agko J, Kiełbik P, Trela M, Witkowska-Piłaszewicz O.
(2026).
Transcriptomic signatures reveal systemic adaptations and immune modulation in response to training and competitive racing in horses.
Equine Vet J.
https://doi.org/10.1002/evj.70154
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